1. Field of the Invention
This invention relates generally to high performance ionomer assisted electrolytic cell electrode structures; a process for preparing such electrode structures; and to an electrolysis cell employing such high performance electrode structures.
2. Description of Related Art
Electrolysis cells and fuel cells are basically energy conversion devices and include liquid electrolyte cells and solid or membrane electrolyte cells. Electrolysis and fuel cells are structurally similar, but are utilized to effect different half-cell reactions. Therefore, each type of cell relies upon different conditions for the maintenance of high ion conductivity within the cell. In particular, electrolysis cells require the movement of appropriate quantities of water to the catalyst sites and the simultaneous movement of gas away from the catalyst sites. Additionally, it is necessary to maintain high ion conductivity within the cell. Fuel cells, in contrast, require that water be repelled from the electrode in order to prevent flooding. Water is continuously drained out of these cells during operation. It is also necessary in fuel cells to provide a high flux of ions to and from the active electrode sites.
Membrane electrolysis cells and membrane fuel cells typically comprise an anode, a cathode, an ion exchange membrane disposed therebetween, for providing ion exchange between the cathode and the anode electrodes, an anode chamber and a cathode chamber. Such cells offer many advantages, including the ability of ion exchange membranes to resist depletion or degradation, the ability to provide positive separation of process fluids and the membranes easy incorporation into electrolysis cell and fuel cell structures. However, as alluded to above, the half-cell reactions that take place at the anode and cathode require catalysts to proceed at useful rates. Accordingly, techniques have been developed to incorporate catalyst materials into membrane electrolysis cells and fuel cells.
Catalyst materials were first incorporated into such cells by hot pressing the materials directly into the surface of the membrane. High catalyst loadings were necessary, however, to achieve useful current densities.
Efforts directed toward reducing catalyst loadings in fuel cells include the use of carbon-based electrode structures that comprise platinum loaded carbon particles on a carbon cloth or carbon paper electrode substrate, bound together by a hydrophobic component such as polytetrafluoroethylene (PTFE) or Teflon.RTM.. Such a structure is disclosed in U.S. Pat. No. 4,876,115, issued Oct. 24, 1989. In addition to hydrophobicity, Teflon.RTM. is taught as providing gas access with the electrode. (See Col. 4, lines 22 to 26.) The catalyzed sides of the carbon electrodes of the '115 patent are impregnated to a depth of about 10 micrometers (.mu.) with a solubilized form of ionomer to increase the access of the electrolyte to the catalyst within the catalyst-C/Teflon.RTM. layer. Application of such ionomer materials is by spraying or by deposit with an applicator onto the surface of the electrode. (See Col. 4, lines 66 to 68.) However, due to varying thicknesses in the catalyst layer, uneven impregnation of the solubilized ionomer in the catalyst layer results. It has been observed that some areas of the impregnated electrode are not fully impregnated while other areas have ionomer material extending so far within the electrode that gas diffusion through the electrode is impeded. It has been further observed that hydrophobic binders, such as Teflon.RTM. block proton and oxygen access to catalyst sites and that differential swelling between the ionomer material and catalyst layer result in delamination with resulting discontinuity in the ion path and decreased cell longevity.
An attempt to overcome these deficiencies is described in U.S. Pat. No. 5,234,777, issued Aug. 10, 1993. The '777 patent is directed to an improved solid polymer electrolyte membrane assembly where the improvement comprises a membrane and a film of a proton conducting material or binder having a supported platinum catalyst uniformly dispersed therein, where the film is bonded to the membrane. The catalyst/binder layer can be fabricated as a separate unit and transferred to the surface of the membrane by hot pressing at temperatures between 125.degree. C. and 145.degree. C. onto the membrane (see Col. 5, lines 3 to 7) or can be directly applied, in the form of an ink, to the surface of the membrane at temperatures of at least 150.degree. C. (see Col. 6, lines 27 to 36). This high temperature application and subsequent drying reportedly cures the catalyst layer.
It has been observed however that painting and baking such liquid coatings or inks results in cracking and shrinkage and therefore voids resulting in discontinuity. In addition, applying the ink directly to the surface of the membrane results in varying thicknesses of the resulting film and small voids due to ink movement during application and nonuniform membrane thicknesses. Moreover, such films degrade and are rendered less hydrophilic when heated to elevated temperatures and, as a result, reduced ion conductivity is observed in electrolysis cells employing such membrane assemblies. Further, it is theorized that the bonding between the catalyst and the ionomer material is weak, where moderate catalyst/ionomer dispersion formation temperatures are employed, thereby resulting in decreased activity at the catalyst/ionomer interfaces. Finally, it is theorized that the absence of a rehydration step in the method for preparing such a membrane assembly further results in a decrease in observed ion conductivity.
Accordingly, it is an object of the present invention to provide ionomer assisted electrode structures that enhance electrolysis cell performance; that demonstrate increased structural integrity; and that have low catalyst loadings.
It is another object of the present invention to provide electrode structures having catalyst ionomer layers where hydrated and swollen ionomer solids are bonded to discrete catalyst particles.
It is yet another object of the present invention to provide electrode structures having a catalyst ionomer layer located adjacent to a hydrated ion exchange membrane that demonstrate improved bonding between the catalyst ionomer layers and the membranes.
It is a further object of the present invention to provide a process for preparing such electrode structures.
It is yet a further object of the present invention to provide an electrolysis cell that employs such electrode structures.